Diagnosis Of Feline Lower Airway Disease

Claire R. Sharp, BSc, BVMS (Hons), MS, Diplomate ACVECC

Lower airway disease is remarkably common in cats, and it poses a clinical challenge whether in its acute or chronic state. In this first part of a 2-article series, the diagnosis of lower airway disease is investigated, from disease overview to presentation to diagnostic modalities.

Lower airway disease (LAD) is remarkably common in cats, with estimates suggesting that 1% of the general cat population is affected. It poses a clinical challenge whether in its acute or chronic state.

DISEASE OVERVIEW

Definition

The term feline lower airway disease is often used to refer to a somewhat heterogeneous group of conditions that affect the lower respiratory tract of cats. Since no consensus definition exists for these lower airway diseases in veterinary medicine, different authors use different terminology, creating some confusion in the literature.

Feline LAD has been referred to as feline asthma, bronchial asthma, allergic asthma, bronchial disease, and chronic bronchitis.

Classification

There are 2 predominant phenotypic categories of lower airway diseases in cats:^1,2

• Asthma
• Chronic bronchitis.

The term asthma suggests reversible bronchoconstriction and predominantly eosinophilic airway inflammation. In contrast, chronic bronchitis is associated with neutrophilic inflammation.¹ These 2 syndromes represent opposite ends of the spectrum of feline LAD; however, some cats have both eosinophilic and neutrophilic inflammation, a condition termed chronic asthmatic bronchitis.³

While clinical signs of asthma and chronic bronchitis are very similar, as is their current management, it is important to differentiate between them due to different underlying pathologies. Current research is aimed at developing more disease-specific diagnostics and therapeutics.²

The Table summarizes the differences between feline asthma and chronic bronchitis.

Predilection

Siamese cats are predisposed to lower airway disease, with a breed prevalence of up to 5%.4 Some authors suggest there is no sex predilection,¹ while others have documented that females are affected more significantly.³
Asthma develops in young to middle-aged cats, with a reported mean age of 4 years (range, 1–15 years).¹ Although some cats are older when they experience an asthmatic crisis, a history of paroxysmal cough suggests that they likely suffered from asthma for some time.

Etiology

Asthma in cats is considered an allergic disease.²

• Environmental aeroallergens implicated as triggers include house dust mites, Bermuda grass allergen, and pollens.^2,5
• Additional presumptive predisposing factors include city environment (higher incidence than cats in rural environments) and smoking households (anecdotal).^2,5

In contrast, chronic bronchitis is believed to result from a previous insult, such as an infection or inhaled irritant that permanently damaged the airways.²

Chronic asthmatic bronchitis is thought to be associated with chronic allergic airway inflammation (ie, allergic asthma is the underlying airway disease) that triggers airway damage, resulting in development of neutrophilic inflammation.²

Pathogenesis

The mechanisms underlying development of feline asthma are postulated to be similar to those of allergic asthma in humans:²

A type I hypersensitivity reaction occurs in the airways. The trigger is thought to be aeroallergen exposure, which induces a preferential polarization of the immune response toward:

• Production of inflammatory mediators from T helper 2 lymphocytes
• Suppression of the T helper 1 immune response.²

Inflammatory mediators promote:

• IgE production and eosinophilic inflammation and infiltration, leading to the hallmark features of allergic asthma: airway inflammation, hyperreactivity, and remodeling.
• Mucus hypersecretion^1,2
• Bronchoconstriction and airway hyperreactivity (ie, sensitivity of airway smooth muscle to stimuli) when released from eosinophil and mast cell granules (eg, histamine).

Airflow limitation results from airway narrowing due to cellular infiltrates, mucus secretion, and airway edema, leading to clinical signs, such as cough and wheeze.

• Acute bronchoconstriction exacerbates airflow limitation and can precipitate severe and potentially life-threatening clinical signs.⁴
• Bronchoconstriction, mediated by airway smooth muscle contraction, likely occurs, at least in part, due to an imbalance between:
  • Mediators of bronchoconstriction (parasympathetic/cholinergic system)
  • Mediators of bronchodilation (sympathetic/adrenergic system and nonadrenergic/noncholinergic system, the latter which includes vasoactive intestinal peptide and nitric oxide)⁵
• Bronchoconstriction in asthmatic cats is reversible with therapy and/or upon removal of the aeroallergenic stimulus.

HISTORY & EXAMINATION

Clinical Signs

Cats with LAD have variable clinical signs, typically presenting with chronic cough or in an acute respiratory crisis.

Chronic cough may be sporadic or noted multiple times a day, and the cat usually behaves normally between coughing episodes.

With acute onset of respiratory distress, a history of cough may indicate asthma as a diagnosis. Signs vary and may include:

• Occasional episodes of respiratory distress interspersed with asymptomatic periods⁴
• Seasonal variation in presence and severity of cough³
• Vomiting after coughing.³

Many owners think that coughing is associated with hairballs, not realizing that a hairball vomited up from the stomach is separate from cough.

Unfortunately, this misunderstanding means owners often dismiss coughing until signs worsen.

Physical Examination

Examination findings depend on chronicity and severity of clinical signs.

Cats with chronic bronchitis may have normal findings or, if critical, moderate to severe respiratory distress (dyspnea and tachypnea).

Cats with asthma have predominantly expiratory dyspnea. Those more severely affected may have a marked expiratory push or even expiratory grunt. Open mouth breathing and/or orthopnea may be noted. Thoracic auscultation may reveal pronounced crackles, wheezes, or increased breath sounds. Auscultable wheezes are characteristic of airway narrowing and airflow limitation that occurs in cats with asthma.

Vital signs can also help differentiate asthmatic cats from those with other causes of respiratory distress, such as heart failure (see Signs of Heart Failure). Cats with LAD are usually normothermic (> 100°F), with a normal heart rate (eg, 180–220 beats/min).

EMPIRICAL THERAPY & DIAGNOSTICS

The diagnostic approach for cats with LAD depends somewhat on the severity of clinical signs at presentation.

Empirical Therapy

Cats with severe acute respiratory distress may be too unstable to safely undergo extensive diagnostic procedures. Rather, these cats benefit from acute empirical therapy, such as oxygen supplementation in an oxygen cage, minimal handling to reduce stress, and bronchodilator treatment trial.

If asthma is suspected, injectable glucocorticoids may be given on an emergency basis prior to determining a definitive diagnosis. A rapid, and near complete, resolution of clinical signs associated with bronchodilator and/or glucocorticoid therapy supports a diagnosis of feline allergic asthma.

Thoracic Radiographs

Abnormalities on thoracic radiographs consistent with allergic asthma include generalized:⁷

• Bronchial to bronchointerstitial pattern (Figure 1): A bronchial pattern is characterized by “doughnuts” and “tramlines”; an interstitial pattern reflects increased opacity of the pulmonary interstitium.
• Hyperinflation (air trapping): On a lateral radiographic projection, evidenced by flattening of the diaphragm, expanded lung fields, hyperlucent lungs, and increased distance from the caudal aspect of the cardiac silhouette to the diaphragm.

Figure 1. Lateral (A) and ventrodorsal (B) thoracic radiographs of a cat with asthma; note the marked, generalized bronchointerstitial pattern.
A

 B

Occasionally cats with asthma will have a collapsed right middle lung lobe, and can also present with spontaneous pneumothorax⁸ (Figure 2) or caudal rib fractures.⁹ A patchy alveolar pattern has also been described in cats with LAD.³

Figure 2. Lateral thoracic radiograph of a cat with asthma and mild secondary spontaneous pneumothorax. Atelectasis of the lungs associated with the pneumothorax has resulted in retraction of the lung lobes from the chest wall and diaphragm, increased interstitial opacity in the pulmonary parenchyma, and the underlying bronchial pattern.

Normal thoracic radiographs do not rule out LAD in cats. However, thoracic radiographs are important as they help exclude other causes of respiratory distress (eg, heart failure, neoplasia).

Cardiac Ultrasound & Echocardiography

Focused cardiac ultrasound can be used to assess the heart and help rule out cardiac causes of respiratory distress.

In emergent situations, evaluation of the left atrial to aortic ratio in a right-sided, short axis view can be a particularly useful test. In cats, the normal left atrial:aortic ratio is < 1.5; LA:Ao ratios of > 2 are very suggestive of heart failure.^10,11

While complete echocardiography is best performed by a cardiologist, noncardiologists can be trained to perform focused cardiac ultrasound for some applications in the veterinary emergency room.¹²

Lower Airway Cytology

Obtaining samples for lower airway cytology is the only way to definitely diagnose LAD. These samples can be obtained by an endotracheal wash or bronchoalveolar lavage (BAL). BAL samples can be obtained bronchoscopically, but blind BALs are performed much more commonly. See Step by Step: Blind Bronchoalveolar Lavage for a brief description of the technique; a more detailed description can be found in the literature.¹³

Airway samples should be submitted for cytology and aerobic culture and susceptibility, including culture and/or polymerase chain reaction for Mycoplasma species.

• Allergic asthma is characterized by predominantly eosinophilic airway inflammation (> ≈17% eosinophils) (Figure 3), although other inflammatory cells may be present.
• Chronic bronchitis is characterized by neutrophilic inflammation; neutrophils are nondegenerate and nonseptic.
• Chronic asthmatic bronchitis is characterized by mixed inflammation with significant eosinophilic and neutrophilic components.^2,3

Endobronchial Evaluation

Bronchoscopy and BAL have been described in cats with LAD.¹⁴ Endobronchial abnormalities were not different in cats with asthma when compared with those found in other lower respiratory tract diseases, notably pneumonia and neoplasia. Nonspecific findings included excessive mucus accumulation, stenosis of bronchial openings and nodular epithelial irregularities, airway hyperemia, airway collapse, and bronchiectasis.¹⁴ I rarely perform bronchoscopy in cats.

Additional Diagnostics

Additional diagnostics recommended in cats with LAD include:

• Routine hematologic and biochemistry tests: In most asthmatic cats, complete blood counts and biochemistry analyses are normal, although a peripheral eosinophilia (> 1500 cells/mcL) may be present.
• Feline immunodeficiency and leukemia virus tests (FIV/FeLV): Recommended in cats that live outdoors or in multicat households as concurrent retroviral disease may affect prognosis.
• Baermann fecal test: Evaluates for potential lung worm infection.
• Heartworm antigen and antibody testing: Should be performed routinely in cats with signs of LAD given the increasing recognition of feline heartworm-associated respiratory disease (HARD).
  • The SNAP Feline Triple Test (idexx.com) detects heartworm antigen, FIV, and FeLV.¹⁵ However, because the test measures female reproductive antigen, cats may test negative if infected with small worm burdens, only adult male worm(s), or larval forms.¹⁶
  • In a cat with consistent clinical signs and a positive antigen test, retest to confirm positive status; a repeat positive test confirms a diagnosis of HARD.
  • In a cat with signs of LAD and a negative antigen test, follow-up with an antibody test. A positive antibody test in a cat with consistent clinical signs suggests current infection with juvenile or adult worms.

Part Two of this article, Treatment of Feline Lower Airway Disease, will be published in an upcoming issue of Today’s Veterinary Practice.

BAL = bronchoalveolar lavage; FeLV = feline leukemia virus; FIV = feline immunodeficiency virus; HARD = heartworm-associated respiratory disease; LAD = lower airway disease

References

1. Venema CM, Patterson CC. Feline asthma: What’s new and where might clinical practice be heading? J Feline Med Surg 2010; 12:681-692.
2. Reinero CR. Advances in the understanding of pathogenesis, and diagnostics and therapeutics for feline allergic asthma. Vet J 2011; 190(1):28-33.
3. Moise NS, Wiedenkeller D, Yeager AE, et al. Clinical, radiographic, and bronchial cytologic features of cats with bronchial disease: 65 cases (1980-1986). JAVMA 1989; 194(10):1467-1473.
4. Padrid P. Chronic bronchitis and asthma in cats. In Bonagura JD, Twedt DC (eds): Current Veterinary Therapy XIV. Philadelphia: WB Saunders, 2009, pp 650-658.
5. Reinero CR, DeClue AE, Rabinowitz P. Asthma in humans and cats: Is there a common sensitivity to aeroallegens in shared environments? Environ Res 2009; 109:634-640.
6. Goutal CM, Keir I, Kenney S, et al. Evaluation of acute congestive heart failure in dogs and cats: 145 cases (2007-2008). J Vet Emerg Crit Care 2010; 20(3):330-337.
7. Gadbois J, d’Anjou MA, Dunn M, et al. Radiographic abnormalities in cats with feline bronchial disease and intra- and interobserver variability in radiographic interpretation: 40 cases (1999-2006). JAVMA 2009; 234:367-375.
8. White HL, Rozanski EA, Tidwell AS, et al. Spontaneous pneumothorax in two cats with small airway disease. JAVMA 2003; 222:1573-1575, 1547.
9. Adams C, Streeter EM, King R, et al. Cause and clinical characteristics of rib fractures in cats: 33 cases (2000-2009). J Vet Emerg Crit Care 2010; 20:436-440.
10. Smith S, Dukes Mc-Ewan J. Clinical signs and left atrial size in cats with cardiovascular disease in general practice. J Small Anim Pract 2012; 53(1):27-33.
11. Abbott JA, MacLean HN. Two-dimensional echocardiographic assessment of the feline left atrium. J Vet Int Med 2006; 20(1):111-119.
12. Tse YC, Rush JE, Cunningham SM, et al. Evaluation of a training course in focused echocardiography for noncardiology house officers. J Vet Emerg Crit Care 2013; 23(3):268-273.
13. Reinero CR. Bronchoalveolar lavage fluid collection using a blind technique. Clin Brief 2010; March:58-61.
14. Johnson LR, Vernau W. Bronchoscopic findings in cats with spontaneous lower respiratory tract disease (2002-2009). J Vet Int Med 2011; 25:236-243.
15. Lorentzen L, Caola AE. Incidence of positive heartworm antibody and antigen tests at IDEXX Laboratories: Trends and potential impact on feline heartworm awareness and prevention. Vet Parasitol 2008; 158:183-190.
16. Dillon AR, Brawner AR, Jr, Robertson-Plouch CK, et al. Feline heartworm disease: Correlations of clinical signs, serology, and other diagnostics—results of a multicenter study. Vet Ther 2000; 1:176-182.

Claire R. Sharp, BSc, BVMS (Hons), MS, Diplomate ACVECC, is an assistant professor and specialist in small animal emergency medicine and critical care at Tufts University Cummings School of Veterinary Medicine. Dr. Sharp is also the Director for the Clinical Reasoning and Small Animal Clinical Procedures courses for third-year veterinary students and is involved in the clinical instruction and mentoring of critical care residents and interns. Her primary clinical interests include the diagnosis and management of systemic inflammatory response syndrome, sepsis, shock, polytrauma, and respiratory distress. Dr. Sharp’s research endeavors have involved identifying clinically relevant biomarkers for the diagnosis and prognostication of emergency room diseases, such as sepsis, feline asthma, and hemangiosarcoma.

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